Separate out linear model component of StaticLinear

[jamesturner246]

The StaticLinear module is currently very bloated, with things such as intervention policy, income class, sector etcetera, also being performed. We need to separate this functionality out into it's own module, with its own separate config.json, which can be called by the mechanism in #383. This will better facilitate modularity, code reuse and testability.

• separate JSON config with loader
• the concise module itself
• interface with module loader

This module will NOT be used in simulations yet. Integration will come in a future PR, after testing and feedback.